1. Field of the Invention
The present invention relates in general to a novel method for the measurement of the Thiele-Small small signal parameters of a moving-coil electromagnetic transducer. The measurement method is based on a time domain analysis of the transient response of the loudspeaker voice coil circuit to a current step of excitation. The damped sinusoidal transient generated by such an excitation is sampled and recorded. The loudspeaker parameters are then calculated from a linear predictive analysis of the recorded data.
2. Description of the Prior Art
Fundamental to the successful low frequency design of any loudspeaker system which employs a moving-coil electromagnetic transducer is a knowledge of its Thiele-Small small-signal electroacoustic parameters. In multiway systems, such as two-way and three-way loudspeakers, the low frequency response of the upper frequency drivers (e.g., the mid-range and tweeter units) is not as important a design consideration as that of the woofer. This is because the upper frequency drivers are always crossed over to a lower frequency driver. In the case of the woofer, however, its performance determines the ultimate low frequency response of the system. Therefore, it demands a more careful electroacoustic design.
The system enclosure or box interacts with the woofer to play an important role in the low frequency performance of the system. The proper design of the enclosure for a given driver requires a knowledge of the driver small-signal parameters. With these parameters, the enclosure parameters such as volume, vent tuning frequency in the case of a vented box system, passive radiator design in the case of passive radiator system, etc., can be determined in such a way that the low frequency system response can be accurately controlled for optimum performance. The driver small-signal parameters which must be known are the free-air frequency of resonance, the electrical quality factor, the mechanical quality factor, and the equivalent volume of air which when compressed has the same mechanical spring constant as the driver suspension.
Conventional techniques for the measurement of the small-signal parameters of a driver are based on sinusoidal steady-state measurements. The equipment required to make these measurements includes an impedance bridge, oscillator, frequency counter, voltmeter, and oscilloscope. Measurements are made at three different frequencies, both in free air and with the driver mounted on a sealed box with a known volume. The parameters are calculated from the data obtained. Because the calculations can involve differences between frequencies that are closely spaced, discrepancies can result. In addition, the accuracy of the measurements can be perturbed by the voice-coil inductance which is neglected in the theory but is present in the measurements. There are accurate sinusoidal techniques which can be used to circumvent the effects of the voice-coil inductance that use series-parallel coupled coils in the voice-coil circuit and AC bridge techniques. These tests using advanced methods are difficult and time consuming and as a result are very costly to perform. Additionally, these advanced methods are difficult to automate.